1. Field of the Invention
The present invention relates to a process for preparing organosilicon compounds by hydrosilylation in the presence of a cyclic ether.
2. Background Art
The addition reaction of unsaturated organic compounds to SiH containing silanes or siloxanes in the presence of a hydrosilylation catalyst has been known for decades and is probably one of the most frequently utilized reaction steps for the synthesis of organofunctional silicon compounds. However, numerous investigations have shown that the hydrosilylation reaction proceeds smoothly and without the formation of by products only in certain cases. Typical side reactions when SiH, carbinol and unsaturated ether functions are present are, for example, dehydrocondensation and acetal formation, which often lead to undesired viscosity increases, up to gelling of the reaction mixture.
For the suppression of side reactions, the prior art discloses various approaches. While the use of basic inorganic compounds, for example alkali metal/alkaline earth metal carbonates, bicarbonates, borates, hydroxides and oxides and also NaH2PO4 and/or Na2HPO4 finds widespread application, U.S. Pat. Nos. 5,191,103 and 6,372,874 recommend the use of basic organic compounds in the form of (sterically hindered) amines or phosphines, and of alkylamines having OH, carbonyl or ether functionality. In contrast, U.S. Pat. Nos. 4,431,789 and 4,847,398 teach the addition of carboxylic acid salts as weak bases. A completely different approach is described by WO 2003037961 A2 and WO 2003037961 A1. In these publications, lactones, lactams and cyclic carbonates and carbamates are preferred as the hydrosilylation additive.
A disadvantage of the processes previously mentioned is that the additives used are for the most part solids which are insoluble in the silicone phase. Especially in a continuous reaction, for example, in loop reactors, tubular reactors or microreactors, this leads to problems, or makes the addition of organic solvents for homogenizing the reaction mixture indispensible. However, the latter constitute an additional disadvantage from an economic point of view, since the removal of the solvent on completion of reaction necessitates a further process step, for example distillation and filtration. It is also known that reactions take place in a heterogeneous system only on the particle surface, so that, to achieve the desired effect, additive amounts in the percent range are necessary.
However, the use of organic bases in the form of amines or phosphines is problematic in that they are known, even in very small amounts, to have an inhibiting action on the hydrosilylation reaction. Accordingly, when the abovementioned amine and phosphine systems are used, larger amounts of catalyst and higher reaction temperatures than usual are required. Incidentally, the situation is also similar for the carboxylic acid salts listed, which often lead to slowing of the hydrosilylation reaction and, in specific cases, even to a reduced selectivity. The most serious disadvantage of the additives specified in WO 2003037961 A2 and WO 2003037961 A1 is their generally poor compatibility with organosilicon compounds. For instance, especially cyclic carbonates and carbamates, and also lactams, are known to have only very sparing solubility in the silicone phase, so that it is necessary to use additive amounts of 1000 ppm and more.
There is therefore a need for a simple and inexpensive process which allows for synthesis of organofunctional organosilicon compounds by hydrosilylation reactions while generating very low levels of by-products.